PROJECT SUMMARY (See instructions) : Neuronal endocytic defects occur in Alzheimer's disease (AD) and Down syndrome (DS). Our previous work has characterized AD-relevant endosomal pathology in DS model systems, and has demonstrated that these endosomal changes lead to pathological dysfunction of the endocytic system and the downstream lysosomal system. This project will investigate the mechanisms of endosomal pathology in the disease, examining the role played by cholesterol and the B-cleaved C-termlnal fragment (BCTF) of the amyloid precursor protein (APP) In modulating endocytic function. With the knowledge base developed previously in this program that the endosomal defects seen in AD and DS are likely to contribute directly to neuronal vulnerability, we will determine in vivo In Aim 1 the mechanisms by which dietary fat and/or cholesterol leads to neurodegenerative changes mediated through endosomal dysfunction. In Aim 2, our understanding of cholesteroldriven early endosomal dysfunction will be extended using primary neurons and human fibroblasts, where the response of endosomal regulatory elements and downstream lysosomal system compartments will be examined. To further our hypothesis that the BCTF of APP is a regulator of early endosomal function, and to examine potential synergy between cholesterol-driven and BCTF-driven endosomal dysfunction, we will probe this BCTF function by employing a novel system using antibodies that bind to BCTFs, increasing cellular BCTF levels. Additionally, the role of the BCTF In mediating endosomal dysfunction following dietary hyperiipidemia will be examined in vivo using BCTF-lacking BACE knockout mice. (3CTF interacting/binding partners will also be further examined in this aim. In part by using a novel X l l a transgenic mouse expressing only endogenous APP but with increased BCTFs and endosomal pathology. Lastly, in Aim 3, we will test our hypothesis that reducing endocytosis using small compound inhibitors of rab prenylation, some of which are FDA approved for other diseases, will decrease neuronal endocytic abnormalities in vivo. This project builds upon our understanding that AD-related endocytic dysfunction Is pathological, can lead to neuronal compromise, and is an important, and potentially mailable, therapeutic target.